Cut the remaining 4 Tablespoons of butter into small pieces and add it to the oats mixture. Using your fingers, rub the mixture together until crumbly.

Sprinkle the mixture evenly on top of the apples and bake for 50 - 60 minutes, until the apples are tender and the topping is crisp. Allow to cool for 10 minutes before serving. Serve with vanilla ice cream, if you wish.